This invention relates to a device for dispensing metered amounts of flowable materials. More specifically, this invention relates to a device for dispensing liquid adhesive materials that includes a sealing arrangement providing for continuous operation over extended periods of time.
Adhesives are used extensively in many industries, such as automotive, medical and electronics industry. Adhesives provide an efficient means of joining together two or more components. In many industrial applications, dispensing devices are typically employed to automatically apply adhesives to products that are being fabricated in a continuous manufacturing process. In certain applications, the adhesives are applied to small objects and in small amounts of only a few drops. If an insufficient amount of adhesive is applied, the two components will not properly bond together. If too much adhesive is applied, the excess adhesive can interfere with the operation of the product or form unwanted bonds with other components. In either case, the incorrect application of adhesive may render the component defective and, therefore, unsuitable for sale.
Dispensing devices in modern industrial processes for effective and efficient operation operate without failure over thousands of cycles of operation. Thus, the dispensing device reliably applies a precise amount of adhesive and operates continuously for long periods of time with a minimum amount of down time for maintenance and repair.
Many of the adhesive dispensing devices used in automated manufacturing processes are mounted on robotic arms. If the dispensing device is too large, it can restrict the movement of the robotic arm and complicate the manufacturing process. As a consequence, adhesive dispensing devices of relatively small size are desirable. However, decreasing the size of the dispensing device creates problems because the smaller chambers and passages of these devices are more likely to become obstructed.
One of the most common problems encountered with adhesive dispensing devices is clogging caused by premature curing of the adhesive before it has been dispensed. This premature curing is often the result of a breach of the sealed vessel that contains the adhesive.
Adhesives often begin to cure when exposed to air or to a material, such as certain metals, that acts as an accelerant. The high performance adhesives used in many manufacturing processes can be especially difficult to handle, because they are usually fast curing, requiring a very brief exposure to air or a curing agent before they harden. As a consequence, these adhesives often begin to cure while still in the dispensing device. Even very small amounts of cured adhesive in the dispensing device can interfere with the operation of the dispenser and require that it be taken out of service.
Many dispensing devices use a valve stem passing through a chamber containing the adhesive to control the outlet flow. The seals that isolate the material inside the device from being contacted by the air on the outside are important components of each device. Known dispensing devices have experienced problems with these seals for a variety of reasons. One of the reasons is the demanding operating conditions. Most dispensing devices used in industrial processes operate at high pressures to insure that the adhesive can be delivered in exact amounts in a short period of time. The long and narrow capillaries in which the adhesives are transported, together with the viscous nature of the adhesives, use operating pressures that can reach or exceed 600 psi in order to assure reliable operation. These elevated pressures shorten the life of the seals. After the seals begin to wear, the high operating pressure causes the adhesive to leak out of the dispenser. Once the adhesive leaks out, it cures and interferes with the operation of the dispenser. The manufacturing process then has to be stopped while the dispensing device is either repaired or replaced. These interruptions are disruptive and expensive for the manufacturer because they effect production and increase maintenance coats.
Dispensing devices with valve type nozzles have also been widely used for dispensing adhesives and other flowable materials. The conventional valve design in such devices has a chamber containing the material that is being dispensed, an outlet nozzle through which the material is dispensed and a needle that opens and closes to regulate the flow of the material. The valve needle is attached to the valve stem which extends through the valve body and connects to an actuating device, which moves the valve stem to open and close the valve. One of the major problems with this design is the leaking of the material at the point where the valve stem passes through the valve body. Conventionally, different types of O-ring seals are used to prevent leakage. While O-ring seals are acceptable for some applications, they have been found to prematurely leak when used in high pressure applications or when corrosive materials are used.
Adhesive materials which can begin to cure while still in the dispensing device present one of the most difficult applications for dispensing devices. The adhesive materials tend to adhere to the valve stem and harden. The hardened adhesive material on the stem will begin to abrade the valve seal over time as the stem moves back and forth through the seal to open and close the valve. In a typical manufacturing operation, the valve may cycle open and closed hundreds of times an hour. Eventually, the abrasion from the hardened adhesive on the valve stem causes the seal to leak and the dispensing device has to be repaired or replaced. In high pressure applications, this problem is exacerbated by the high pressures which place greater stress on the seals and cause the materials to leak more quickly.
The increased maintenance costs and the lost production time required for repair or replacement of inoperable dispensing devices, have created a need for a more dependable dispensing device.
The present invention, therefore, provides a dispensing device for dispensing flowable materials, such as adhesives. The dispensing device includes a housing having a chamber and an inlet port and an outlet port for receiving and dispensing a flowable material. A valve assembly is supported within the housing having a moveable valve stem for opening and closing the outlet port. A flexible sleeve is circumferentially disposed in surrounding relationship about a section of the valve stem and spaced apart from the valve stem for isolating the flowable material from a section of the valve stem.
In a preferred embodiment, the flexible sleeve includes a compressible section in the form of a bellows. The flexible sleeve defines a space about the valve stem. The dispensing device includes means for introducing a liquid into this space. Means is also provided for controlling the pressure of the liquid to compensate for pressure perturbations resulting from the opening and closing of the valve.
The sleeve can be made from a variety of materials, including polytetrafluoroethylene (PTFE), nylon, polyethylene, polypropylene and combinations thereof. The most preferred material is PTFE, which has been found to be chemically resistant to a variety of adhesive materials while providing the durability and strength needed for continuous operation over extended periods of time. In addition to being durable, the material used for the sleeve must also be flexible so that it will be able to change its shape as the liquid is withdrawn and then reintroduced when the valve opens and closes.
The dispensing device includes actuating means for moving the valve stem to open and close the valve and sealing means disposed between the valve chamber and the actuating means. The sealing means prevents the material in the valve chamber from leaking out at the point where the valve stem passes through the valve body. The sealing means can include a seal having a tapered nozzle. The sleeve is disposed around the nozzle and the seal is disposed between the flowable material and the actuating means. In another embodiment, the seal also includes means for introducing a liquid between the sleeve and the stem.
The present invention provides a number of advantages over dispensing devices used in the prior art, including longer life and more efficient operation. This design isolates the surface of the seal that contacts the valve stem from the material in the valve chamber and insures that the valve stem can pass through the seal without experiencing clogging problems common in prior art devices. By isolating the valve stem from the seal, the present invention avoids the problems caused by adhesives hardening on the valve stem and, thereby, increases the life of the seal. The section of the valve stem that passes through the valve body is surrounded by a liquid so that the material in the valve chamber does not contact the valve stem at the point where it passes through the seal. This minimizes the wear on the seal and allows the valve to cycle on and off for greater periods of time before requiring maintenance or replacement. Isolating the valve stem from the seal also minimizes clogging and provides more reliable operation. As a result, the dispensing device can be continuously operated for longer periods of time than prior art devices and, thereby, provides increased production at reduced costs.